1. Field of the Invention
This invention relates to a thermal barrier and more specifically to a thermal barrier for a poppet valve used in a combustion chamber of a piston engine.
2. Description of the Prior Art
Shields or thermal barriers for protecting poppet valves in piston engine combustion chambers are well-known. For many years such shields or barriers have been proposed for the purpose of reducing heat corrosion and weakening of the valve head, to improve or enhance combustion, to reduce heat conduction through the valve, etc. These and other purposes are provided by a poppet valve thermal barrier disclosed in the above mentioned U.S. Pat. No. 4,300,492. Therein, the thermal barrier includes a cup-shaped sheet metal cap having a flat shield portion spaced from the valve face and a cylindrical skirt portion welded to the valve head to define evacuated chamber. The chamber may be filled with an insulating material. The skirt portion, which is conductively connected to the relatively hot shield portion and the relatively cool valve head, has a temperature gradient over its length between the shield portion and valve head or weld. This gradient and radially outward forces from thermal expansion of the shield portion causes the skirt portion to expand varying amounts over its length in bellmouth fashion to relieve the stresses. In some applications the stresses may exceed desirable levels in spite of the bellmouth expansion of the skirt portion. Further, in some applications the flat shield portion may lift off of the face portion or the insulation disposed between shield portion and the face portion, whereby the shield portion oil-cans and fatigues due to the cyclic pressures in the combustion chamber. The lift-off is believed to be caused by stresses at the intersection of the shield and skirt portion.